Method for fabricating turbine assembly



y 16, 1951 D. J. BLOOMVBERG 2,983,992

METHOD FOR FABRICATING TURBINE ASSEMBLY Filed Feb. 7, 1957 INVENTOR.David J B/oomberg Jim J AWL.

ATTORNEY United States Patent i METHOD FOR FABRICATING TURBINE ASSEMBLYDavid J. Bloomberg, Newton, Mass, assignor to David J. Bloomberg, Inc.,Newton, Mass, a corporation of Massachusetts Filed Feb. 7, 1957, Ser.No. 638,789

7 Claims. (Cl. 29- 1563) My invention relates to improvements in methodsof turbine fabrication and to turbines fabricated according to saidimproved methods.

More particularly, my invention relates to improved methods offabrication of turbine bucket or nozzle assemblies and improved methodsof fastening bucket assemblies to turbine wheels.

Still more particularly, my invention relates to improved ways ofpreheating bucket assemblies for mounting on wheels, said preheating andmounting being accomplished without damage to the buckets or appreciableoxidation thereof, thereby permitting secure fastening of the buckets towheels cast in contact therewith.

For reasons such as proper stress distribution, it is often desirable tomake turbine buckets of material which is difierent from that which isemployed in the turbine wheels. This fact necessarily implies that awheel and its buckets often cannot initially be made in one piece, andthat the buckets must somehow be joined to the wheel. Such joints mustnecessarily be very strong in order to be capable of standing thestresses which exist in turbines at high speeds. In the past, bucketshave been fastened mechanically to the wheel, but such a method offastening becomes increasingly inconvenient and ditficult in view of twoconsiderations. The first of these considerations is the demand forrapid mass production of turbinm, especially gas turbines and specialturbines for missiles. The second consideration is the fact thataviation turbines demand light weight, and the additional fact that oneway to lighten the turbine is to make the wheel disc as thin aspossible, and the buckets as narrow as possible. Not only does narrowingthe buckets tend to increase the difiiculty of fastening the buckets tothe wheel, but also such narrowing of the buckets tends to aggravate anytendency of the buckets to vibrate.

One way to handle the problem of fastening the buckets to the turbinewheel is to place the buckets in position around the periphery of acircle and then pour molten metal into a properly shaped mold, such thatthe molten metal forms a wheel and fuses the innermost ends of thebuckets, thereby fastening the buckets to the Wheel as formed by themolten metal. In order to permit the molten metal to fuse the ends ofthe buckets upon contacting them, the buckets themselves must bepreheated to a temperature not too far below that of the molten metal.In other words, in order to get a satisfactory fused joint between thebuckets and the wheel, the buckets must be prevented from conductingaway too much of the heat from the molten metal. Such excessive heatconduction is prevented by preheating the buckets to a temperatureapproaching that of the molten metal. However, preheating the bucketsfor any appreciable length of time in the atmosphere tends to produceoxidation of the buckets, and any serious oxidation of the ends of thebuckets prevents the formation of a satisfactory fused joint between thebuckets and the molten metal. It has been attempted to prevent thisoxidation by pre- Patented May 16, 1961 heating the buckets by means ofinduced high-frequency electric currents and by making the heatingperiod so intense and brief that no serious oxidation can take place.

' The results of this method have not been entirely satisfactory. Onedefect of this method is the fact that a short, intense period ofpreheating the buckets themselves may alter the crystalline structure ofthe bucket shanks in such a way as to impair the strength of the metalin a place where strength is of the utmost importance.

Accordingly, it is an object of my invention to provide a method wherebyturbine buckets may be preheated for attachment to a molten wheelwithout said turbine buckets becoming seriously oxidized or impaired instrength.

It is another object of my invention to provide a way in which turbinebuckets or nozzle diaphragms may be conveniently pre-cast and maintainedin proper position for further fabrication operations.

It is still another object of my invention to provide turbine wheelassemblies which are strong, light, and resistant to vibration.

It is a still further object of my invention to provide a turbinefabrication method which is simple, convenient, and capable of a highdegree of automatization.

Briefly, I am able to fulfill these and other objects of my invention bypre-casting an assembly of buckets arranged around the periphery of acircle, each bucket being integral with the adjacent portion of a pairof rings cast at the same time as the buckets, one ring being positionedon either side of the shanks of the buckets. Preheating of the bucketspreparatory to the pouring of a cast wheel is accomplished by inducing ahigh-frequency electric current in the rings, whereby the buckets arepre-heated indirectly by thermal conduction from the rings, thepreheating process being carried out in a vacuum or an inert atmosphere.The rings, which are expendable, may later be machined away, leaving thebuckets securely joined to the wheel cast in contact with them.

For a full understanding of my invention, reference is made to thefollowing complete specification, taken in conjunction with the appendedclaims and the accompanying drawings, wherein:

Fig. 1 is a fragmentary view of a pre-cast assembly of turbine bucketsand expendable preheating rings;

Fig. 2 is a modified sectional view taken through a bucket and itspreheating rings, and showing by dash lines the position, relative tothe bucket, where the turbine wheel may be cast;

Fig. 3 is a modified sectional view of a mold in which the wheel may becast, showing associated apparatus and showing the position of thebuckets and rings during casting of the wheel; and

Fig. 4 is a modified sectional view through a portion of a turbine wheelassembly, showing damping rings which may be installed for minimizationof bucket vibration.

In order to facilitate a clear explanation of my invention, it may behelpful first to offer a brief explanation of the terms to be used inthe specification. For the purpose of the explanation, reference will bemade to Fig. l of the drawings.

A finished turbine wheel consists of a more-or-less discshaped objectwith a large number of buckets attached around its periphery. Properlyspeaking, the wheel is the disc-shaped object alone, without thebuckets. The word wheel will be used in that sense in the followingparagraphs. Sometimes one may find the word wheel used to mean theassembly of disc and buckets, but this specification will use the wordswheel assembly if both the disc and the "buckets are meant to beincluded. Each \mit good thermal contact.

'niques.

the wheel and shaft is designated generally by the nu- 3 bucket consistsof a blade 11, a base 12 of blade 11, and a shank 14, as shown in Fig. 2and Fig. 4, as well as in Fig. 1. The modified sectional views of Fig. 2and Fig. 4 are such as would be obtained if a plane were passed througha bucket, the longitudinal axis of the bucket being in the plane. Theorientation of the plane is also such that, when the bucket is installedon a turbine wheel, the plane would pass through the radius of the wheeland would be normal to the wheel. The two shaded portions, 16 and 17, ofFig. 2 represent cross sections of the expendable heating rings, andhave been shaded in order to distinguish the rings from the buckets andfrom the space to be occupied by the turbine wheel. The cross sectionsof the bucket and of the rings are, of course, formed by the same plane.

As for the method by which the pre-cast assembly of Fig. 1 may beproduced, it is suflicient to say that the assembly may be produced bymeans of familiar techniques, such as by pouringthe assembly in aspecial shell mold. In preparing the shell mold, care should be taken.that the bucket spaces are regularly arranged, with their inner andouter extremities on respective concentric circles. Likewise, careshould be taken that the portions :'of the mold which are to shape faces18 and 19 of the respective rings are accurately formed, for the turbinewheel is to be poured in contact with those faces. The ring-definingcavities in the mold may serve as headers, through which molten metalmay be passed to form the buckets in the bucket-defining cavities of themold.

-The amounts by which the rings 16 and 17 overlap the buckets and theturbine wheel should be suflicient to per- In particular, it isimportant '.that rings 16 and 17 overlap shanks 14 of the bucketssufficiently to permit good heat transfer from the rings to the .bucketsduring the preheating operation. The pre-cast assembly of buckets andtwo rings forms an entity which is easy to make and easy to handle afterit has been made.

After the pre-cast assembly of buckets and rings is :removed from themold in which it was cast, the assembly is placed in a second mold andmay then appear as in Fig. 3, which is a modified sectional view of themold in which the turbine wheel is to be cast. In Fig. 3, the portion ofthe pre-cast assembly which is diametrically op- 26 is a portion of thesame ring shown in section at 16,

while the section of the ring shown at 27 is a portion of the same ringshown in section at 17. The mold has a lower portion 31, an upperportion 32, and a case 33. Once again, the mold comprising portions 31and 32 may be a shell mold manufactured by conventional tech- The cavityfor receiving molten metal to form meral 35. Guides, not shown in Fig.3, would be provided in order to support the mold in case 33.

A pair of induction-heating coils 41 and 42 are shown in Fig. 3, whereininduction-heating coil 42 may be retained in position by a series ofsupports such as those shown at 43. Induction-heating coils 41 and 42should be positioned as near to the expendable preheating rings as thedesign of the mold will permit, thereby providing for maximum magneticflux linkage between the coils and the rings. Induction-heating coils 41and 42 may be cooled by water flowing through the hollow interiors ofthe respective coils. For reasons of safety, the induction-heating coilsshould preferably be outside the confines of the mold because, if moltenmetal were to come the enclosure of the mold. I prefer to employinductionheating coils which are circular, and which are mounted inplanes parallel to those defined by the expendable preheating rings.Thus, when high frequency voltages are applied respectively to coils 41and 42 through electrical leads shown schematically at 45 and 46, eachcoil induces a heating current in the ring adjacent to it. Each ringbehaves in a manner comparable to the short-circuited secondary windingof a transformer of which the, corresponding induction-heating coil isthe primary winding. While induction-heating coils 41 and 42 might eachcomprise a number of turns, I prefer to employ coils each of whichcomprises a single turn. It will be understood that eachinduction-heating coil should be served by a pair of electrical leads,which should preferably be connected to 'the coil at points locatedclose together, thus insuring uniform heating of the preheating rings.The high-frequency voltage may be obtained from any suitablecommercially-available high-frequency generator. When thepre-castassembly is placed in the mold, and a high-frequency potential (of theorder of 500 kilocycles per second, for instance) is applied to theinductionheating coils, the preheating of the pre-cast assembly begins.While the induction-heating coils are hollow and have cooling waterflowing in them, the preheating rings become very hot and supply heat bythermal conduction to the shanks of the turbine buckets adjacent tothem. Inasmuch as the rings are integral with the buckets, and inasmuchas there are two rings supplying heat to the rings may reach levels suchthat the strength of the rings may reach levels such that the strengthof the rings is impaired, the temperatures of the buckets do not reachlevels sufficiently high to damage the buckets. One way to prevent thetemperatures of the buckets from reaching high levels is to form thebuckets so that adjacent buckets are not in contact and, therefore, donot form a continuous path for an induced circulating current. Bucketsmay, if desired, be pre-cast with removable ceramic investments betweenthem to prevent contact. The fact that the rings may suffermetallurgical damage does not matter, because the rings are to bemachined away after the turbine wheel has been cast and after the ringshave served their purpose. It will be noted that the rings overlap theshank ends of the buckets and embrace part of the space into which thewheel is cast.

Now, inasmuch as the buckets are preheated by thermal conduction ratherthan by induced currents flowing in the buckets themselves to any greatextent, it is well to allow more time for the preheating process thanwith prior-art types of preheating processes. Therefore, according to myinvention, oxidation of the buckets is prevented by carrying out most ofthe preheating process under vacuum conditions. In Fig. 3, I have showna vacuum cap 48, which permits the mold to be evacuated and to remainsealed during the preheating period. The atmosphere in the mold may bedrawn off through vacuum lines shown schematically at 49 and 50. Byleaving a small amount of clearance between the mold and the pre-castassembly, I am able to permit the air in the mold to get past thepre-cast assembly to the vacuum lines without also having the moltenmetal get into the vacuum lines. If there is a clearance ofapproximately 10 mils between the mold and the top of the pre-castassembly, the air will be able to pass through the clearance withoutbeing followed by the molten metal to be poured into the mold cavity 35If mold 31 is a shell mold with relatively thin walls, it is desirableto equip the outer surface of the mold with ribs, thereby making themold sufficiently rigid to withstand without substantial deformation thepressure of the atmosphere after the air has been evacuated from theinterior of the mold. It is also desirable to provide a gasket of somesort between the mating edges of the lower portion 31 and-the upperportion. 32 of the mold.

Although vacuum cap 48 is removed from the mold 'during the pouring ofmolten metal into the mold, the period of time during which the bucketsare thereby exposed to the air is comparatively short. If the vacuumconditions are maintained in the mold during the preheating process, thebrief period when the vacuum is spoiled just prior to the pouring of themolten metal into the mold will not be sufiicient to permit seriousoxidation of the buckets.

The same source of vacuum as was used in evacuating the air from themold may also be used for the purpose of drawing the cooling water outof induction-heating coils 41 and 42 immediately after the end of thepreheating period and before the introduction of molten metal into themold. Such withdrawal of the cooling water would be particularlyimportant if the inductionheating coils were placed within the cavity ofthe mold. As aforementioned, I prefer to place the induction-heatingcoils just outside the mold rather than within it, thereby avoiding anyrisk of sudden generation of steam within the coils if residual coolingwater should remain in the coils. By choosing to evacuate the air fromthe mold through vacuum lines which are connected to the mold near itsouter periphery, I not only achieve convenient and safe evacuation ofthe mold but also am able to draw the molen metal into all the portionsof the mold which I desire to fill with metal. For this purpose,evacuation of air through the vacuum lines may be continued during thepouring of molten metal, even though at that time vacuum cap 48 is notin place on the mold.

The process according to my invention is suited to a high degree ofautomatization, in that a series of molds,

each carrying a pre-cast bucket-and-ring assembly, may

be carried on a moving belt or conveyor, passing under a kettle ofmolten metal from which their respective wheel cavities are successivelyfilled. Of course, each of the pre-cast assemblies would be preheated tothe proper temperature by the time it reached the kettle, and thepreheating would be done under no-oxygen conditions. Such an arrangementcould be achieved by using flexible leads to the induction heater orhigh-frequency generator, flexible cooling-water lines, and flexiblevacuum lines. It will be appreciated that, while this discussion hasassumed that bucket oxidation is to be prevented by preheating of thebuckets under vacuum, oxidation of the buckets might also be preventedby preheating the buckets in an inert gas such as argon.

Whether one considers the process of fabrication, the apparatus employedin the fabrication, or the nature of the finished turbine wheel, anumber of modifications may be made without departing from the scope ofmy invention. For instance, the length of time during which preheatshould be applied to the buckets will depend upon the desiredtemperature differential between the preheated buckets and the moltenmetal which is to form the turbine wheel. The desired temperaturedifferential may in turn depend upon the nature of the metals which areemployed for the buckets and for the wheel, respectively. If differentsteel alloys are employed for the buckets and for the Wheel,respectively, the temperature of the molten wheel metal may be about2400 degrees F., and it may be desired to preheat the buckets to atemperature of the order of 1700 degrees F.

As for the apparatus employed in the fabrication, it is clear thatautomatic controls may be employed for purposes such as starting andstopping the conveyor carrying the molds, and for applying and cuttingoff the preheating current, cooling water, and the vacuum or inert gas.Similarly, automatic apparatus may be employed for removing the vacuumcap from the mold and for tapping the kettle of molten metal. Ifdesired, the kettle, as well as the turbine buckets, may be heated bymeans of high-frequency induction apparatus.

If automatized apparatus is employed for controlling the preheating andpouring steps in each one of a series of molds, the controls should becapable of going through the following sequence of steps for each moldinto which a pre-cast assembly of buckets and rings has been placed:

(1) Apply vacuum to the mold.

(2) Apply cooling water and a high-frequency potential to theinduction-heating coils.

(3) Cut off high-frequency potential and cooling water when preheatinghas been accomplished and mold has been placed under kettle.

(4) Remove vacuum cap from mold and fill mold with molten metal fromkettle.

If a mold conveyor is employed, provision should be made forintermittent motion of the conveyor, whereby each mold may dwell beneaththe kettle long enough for the pouring to be completed. Of course, itmight for some purposes be preferable to keep the molds stationary andto move the kettle into the various pouring positions.

With regard to the various modifications which may be made in the bucketassembly and in the finished turbine wheel assembly, it may be pointedout that there may conceivably be occasions when one may not wish tocast the buckets and the preheating rings integrally. If such occasionsshould arise, the buckets and the .preheating rings may be separatelyprepared and may be clamped or welded together for placement in the moldfor preheating and for pouring of the turbine wheel.

The turbine wheel assembly may be modified, within the scope of myinvention, by providing means as illustrated in Fig. 4 for the dampingout of any residual tendency toward bucket vibration. The configurationof Fig. 4 resists bucket vibration by means of a pair of ductile dampingrings 52 and 53 which are respectively installed in notches in theinner'faces of the two sides 12 and 13 of the turbine bucket bases. Suchdamping rings (two rings per wheel assembly) contact all the turbinebuckets and, by friction against the individual buckets, substantiallydamp out vibration of the buckets. In order to take maximum advantage ofdamping rings 52 and 53, the buckets may be designed in such a way thatthe shanks are comparatively long and flexible and the damping rings areinstalled at the outer extremities of the shanks, thereby maximizing theradius of the damping rings. Inasmuch as the damping rings are held innotches on the inner sides of the turbine blade bases, the rings areforced more and more firmly into place as the speed of the turbineincreases. Consequently, the frictional damping action increases as theturbine speed and tendency toward vibration increases. Being formed ofductile metal, the damping rings are capable of stretching to the degreewhich is necessary 0 maintain close contact with the turbine-bucketblade ases.

As has been previously stated, preheating rings which are integrallypre-cast with the turbine buckets may be machined away after they haveserved their multiple purpose. Fig. 4 represents the appearance of aportion of a turbine wheel assembly after the preheating rings have beenremoved. The turbine wheel 57 is fused to the bucket shanks as shown at58, and the preheating rings no longer appear in their respectivepositions on either side of fused joint 58. It has been shown thatpre-cast preheating rings serve the following purposes:

1) To serve as headers through which molten metal can flow during thecasting of the buckets.

(2) To hold the buckets in proper relative positions after thepro-casting operation and prior to the casting of the turbine wheel.

(3) To serve as short circuits for carrying heating currents, and totransfer the heat to the shanks of the buckets for preheating thebuckets without metallurgically damaging them.

Although the foregoing disclosure has been in terms semblies.Accordingly, I intend to cover by means of the appended claims all thosemodifications of turbines 'and fabrication processes which are properlywithin the "scope of my invention.

What I claim as novel and desire to secure by Letters Patent of theUnited States is:

1. A process for the fabrication of a machine assembly ;'comprising thesteps of prefabricating a subassembly including a plurality ofnon-contiguous components and atleast one electrically conductiveexpendable annular member in thermally conductive relationship 'witheach :of said components, preheating said components by in- '-ducing anelectrical current in said annular member and allowing heat to flowfromsaid annular member into said components, casting a central unifyingstructure and simultaneously efiecting fusion of said heated componentsto said central unifying structure, and thereafter removing anexpendable portion of said annular member.

2. A process according to claim 1, wherein said preheating step iscarried out in an environment having a concentration of oxygen smallerthan that of atmospheric air.

3. A process according to claim 1, wherein said pre- 4. A processaccording to claim 5. A process for the fabrication of a turbine wheelassembly including a plurality of non-contiguous buckets and at leastone electrically conductive expendable 'an-' a 1, wherein saidpreheating step is carried out in an inert gas.

sembly comprising the. steps of prefabricating a subaspendable portionof said annular member.

nular member in thermally conductive relationship with each of saidbuckets, preheating said buckets by inducing an electrical current insaid annular memberand allowing heat to flow from said annular memberinto said buckets, casting a central turbine wheel and simultaneouslyeffecting fusion of said heated buckets to said central turbine wheel,and thereafter removing an ex- 6. A process according to claim 5,wherein said buckets are preheated substantially out of contact withoxygen.

7. A process for the fabrication of a turbine wheel assembly comprisingthe steps of prefabricating a subassembly including a plurality ofnon-contiguous alloysteel buckets and a pair of electricallyconductiveexpendable alloy-steel rings in thermally. conductiverelationship with each of said buckets, preheating said buckets byinducing electrical currents in said rings and allowing heat to flowfrom said rings into said buckets, casting a central turbine wheel andsimultaneously effecting fusionof said heated buckets to said centralturbine wheel, and thereafter removing an expendable portion of'each ofsaid rings.

References Cited in the file of this patent UNITED STATES PATENTS1,005,736 Wilkinson l Oct. 10, 1911 2,368,296 Goran Jan. 30, 19452,438,721 Spencer Mar. 30, 1948 2,502,373 Galloway Mar. 28, 19502,530,853 Brennan Nov. 21, 1950' 2,709,568 Thielemann May 31, 19552,757,901 McVeigh Aug. 7, 1956 1 2,770,033 Zarth a Nov. 13, 1956 FOREIGNPATENTS 316,855 Switzerland Oct. 31, 1956 Canada Sept. 8, 1953

